Sand requiring a high water cement ratio, belongs to:
Correct Answer: D. Zone IV
📚 Detailed Explanation: Sand Zone and W/C Ratio
IS 383 classifies fine aggregate into four grading Zones (I to IV) based on particle size distribution. Zone I is the coarsest and Zone IV is the finest. The fineness of sand directly affects the water demand of the concrete mix.
Why D (Zone IV) is correct: Zone IV sand is the finest, with the highest specific surface area (surface area per unit mass). More surface area requires more water to coat and wet all particles. Therefore, Zone IV sand has the highest water demand and requires the highest w/c ratio to achieve workable concrete. Zone I (coarsest) has the lowest specific surface area and lowest water demand, requiring the lowest w/c for equivalent workability. Zones II and III are intermediate. This is why IS 456 cautions that Zone IV sand requires special care in mix design.
IS 383 Sand Zones and W/C Demand
| Zone | Sand Fineness | Specific Surface | Water Demand / W/C |
|---|---|---|---|
| Zone I | Coarsest | Lowest | Lowest |
| Zone II | Medium coarse | Medium low | Low |
| Zone III | Medium fine | Medium high | Medium |
| Zone IV | Finest | Highest | Highest |
Key Concepts for Students
- Zone IV sand may be used in concrete only if the mix is properly designed — IS 456 recommends avoiding Zone IV for reinforced concrete wherever possible.
- Finer sand also increases the risk of drying shrinkage and surface cracking due to the higher paste content needed.
- Fineness Modulus (FM): Zone I FM = 3.5–4.5 (coarsest); Zone IV FM = 0.7–1.5 (finest).